The Free On-line Dictionary of Computing (19 January 2023):

ACIS

    Andy, Charles, Ian's System.

   A geometric engine that most CAD packages now use.  ACIS
   uses a sophisticated object-oriented approach for modelling,
   the data is stored in boundary representation.  Acis is
   owned by Spatial Technologies.

   [How does this differ from "solid modelling"?].

   (1996-03-21)